In 1979 China instituted the one-child policy, which limited every family to just one offspring in a controversial attempt to reduce the country's burgeoning population. The strictly enforced law had the desired effects: in 2011 researchers estimated that the policy prevented 400 million births. In a new study in Science, researchers find that it has also caused China's so-called little emperors to be more pessimistic, neurotic and selfish than their peers who have siblings.

Psychologist Xin Meng of the Australian National University in Canberra and her colleagues recruited 421 Chinese young adults born between 1975 and 1983 from around Beijing for a series of surveys and tests that evaluated a variety of psychological traits, such as trustworthiness and optimism. Almost all the participants born after 1979 were only children compared with about one fifth of those born before 1979. The study participants born after the policy went into effect were found to be both less trusting and less trustworthy, less inclined to take risks, less conscientious and optimistic, and less competitive than those born a few years earlier.

“Because of the one-child policy, parents are less likely to teach their child to be imaginative, trusting and unselfish,” Meng says. Without siblings, she notes, the need to share may not be emphasized, which could help explain these findings.

Only children in other parts of the world, however, do not show such striking differences from their peers. Toni Falbo, a social psychologist at the University of Texas at Austin, who was not involved in the study, suggests that larger social forces in China also probably contributed to these results. “There's a lot of pressure being placed on [Chinese] parents to make their kid the best possible because they only had one,” Falbo says. These types of pressures could harm anyone, even if they had siblings, she says.

Whatever its cause, the personality profile of China's little emperors may be troubling to a nation hoping to continue its ascent in economic prosperity. The traits marred by the one-child policy, the study authors point out, are exactly those needed in leaders and entrepreneurs.